The present invention relates to a differential pulse-code modulation (DPCM) system and, more particularly, an adaptive DPCM (ADPCM) system for performing frequency band compression of speech or like signals.
A DPCM system utilizing redundancy of a speech signal is a band compression system in which the prediction of the amplitude of each sample of the speech signal at the present time point is made on the basis of the past speech signal sample. The basis for such a prediction is that speech signal samples have a high degree of correlation with each other. The simplest DPCM method is to use as the predicted value either the amplitude value of the preceding sample or the product of that preceding sample and a value slightly smaller than 1. A DPCM system improves the signal to noise ratio (S/N) by about 6 dB (decibel) over a PCM system when speech signals are transmitted with the same number of bits. For the same S/N ratio, a DPCM system can save about 1 bit per sample as compared with a PCM system.
As a practical matter, a plurality of the past samples as well as one past sample may be used for the purpose of band compression. More in detail, a predicted value X.sub.j of a speech signal (sample) X.sub.j at a time point j is given by: EQU X.sub.j =A.sub.1 .multidot.X.sub.j-1 +A.sub.2 .multidot.X.sub.j-2 + . . . A.sub.n .multidot.X.sub.j-n ( 1)
where A.sub.1, A.sub.2, . . . , A.sub.n are called the prediction coefficients and are so selected as to lessen the difference between X.sub.j and X.sub.j, i.e., a prediction error. Once the prediction coefficients optimum for the speech signal are selected, an adequate increase of n (about 5 to 8) improves S/N ratio by approximately 10 dB compared with the PCM system. The characteristics of a speech signal vary with time, so that the optimum values of the coefficients also change. Therefore, if the optimum prediction coefficients are selected adaptively to the time-variation of the speech signal, the S/N ratio can be improved by approximately 14 dB. This improvement can be similarly achieved for other signals lying within the bandwidth of a speech signal, such as signals from a data modem (modulator-demodulator) equipment by using the DPCM system.
The prediction coefficients are obtained by the following two methods: one is to analyze a speech signal for the optimum prediction coefficients and the other is to adaptively correct the prediction coefficients so as to lessen the prediction error while the prediction error is being observed. The former method must transmit the quantitized prediction error signal and the prediction coefficients obtained. The latter method need not transmit the prediction coefficients, resulting in simplifying the circuit structure in the system. An ADPCM system using the latter method is discussed by David L. Cohn et al. in his paper entitled "The Residual Encoder-- An Improved ADPCM System for Speech Digitization", IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-23, No. 9, September issue, 1975, pp. 935-941. However since the ADPCM system is vulnerable to transmission errors, the system needs extra hardware to eliminate the error, deteriorating S/N ratio and making the system complicated and costly to manufacture.
Accordingly, one object of the invention is to provide an ADPCM system with a simple circuit construction which is stably operable with a great improvement of S/N ratio.